Cathode interface impedance measurement



y 1957 H. M. WAGNER 2,793,343

CATHODE INTERFACE IMPEDANCE MEASUREMENT Filed Aug. 25, 1953 2 Sheets-Sheet l PULSE GENERATOR a 2s ATTENUATOF mmcnoa 28 32 DIODE CONNECTED PULSE TUBE GENERATOR J f 2 36 DIFFERENCE AMPLIHER OATH-ODE 39\ FOLLOWER FIG. 2 f

INVENTOR.

HEREER M. WAGNER ATTORNEY y 21 5 H. M. WAGNER 2,793,343

CATHODE INTERFACE IMPEDANCE MEASUREMENT Filed Aug. 25, 1953 2 Sheets-Sheet 2 INVENTOR.

HERBERT M. WAGNER I MW? ATTORNEY CATHODE INTERFACE IMPEDANCE IWEASUREMENT Herbert M. Wagner, Asbury Park, N. .l., assignor to the United States of America as represented by the Secretary of the Army Application August 25, 1953, Serial No. 376,538

10 Claims. (Cl. 324-40) (Granted under Title 35', U. S. Code (195-2), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention relates to electrical measuring circuits and more particularly to a circuit for measuring cathode interface impedance.

Electron tubes in many applications are found to be inoperative in certain equipments because of a loss of emission and transconductance. The true nature of many of these failures has been identified as cathode interface impedance and is now recognized in tube design and usage.

Electron tubes having oxide coated cathodes may develop electrical impedance at the cathode during periods of operation which interferes with proper performance of the tubes. Essentially, this impedance is concentrated in a thin interface layer between the metal cathode base and its oxide'coating. For example, in a cathode comprising a nickel base and .a barium-strontium oxide coating, the presence of an activator constituent in the base nickel such as silicon results in a highly resistive interface layer of barium orthosilicate.

The general effects of interface impedance on tube operation are comparable to those of a by-passed resistance in series with the cathode connection of a normal tube without interface impedance. This impedance varies with tube life and operating conditions instead of remaining constant as in the case when fixed components are purposely employed in a circuit. The effects of a by-passed resistance in series with the cathode of an amplifier tube are to increase grid bias and to introduce signal degeneration which attenuates the lower frequencies where the by-passing is ineffective. When the tube is used in a Class A amplifier, the effect of an increased bias is to lower plate current, power output, transconductance, and gain. The signal degeneration varies with frequency and affects the fidelity of the amplifier.

It has been determined that the simplest network that approximates interface impedance is a resistance shunted by a capacitance and in series arrangement with acat-hode lead. -This impedance is characterized by its tendency to build up more rapidly during. standby or cutoff operation with a heated cathode, than during normal tube conduction. in pulsed applications interface impedance causes waveform distortion and reduced pulse amplitude output. For example, when a voltage pulse of rectangular waveform is applied to a tube having interface impedance, the resulting plate and cathode currents. show distortion of the applied pulse. The shape of the distorted pulse has the general appearance of an exponent'ial curve such as is characteristic of resistancecapacitance networks having a time constant from about a tenth to several microsecond-s.

The reliability of :a wide variety of electronic equipment can be affected by interface impedance. Such equipment includes computers, radar, television. and com.- municatio'ns sets and involves circuits su'ch as-multivibrator-s, blocking oscillators, gates, and amplifiers. The

nit-ed States Patent "ice effect is particularly evident in applications where the tube may be at cutoff for appreciable periods. Failures may be caused by a distorted frequency response, low power output, timing errors, failure to trigger, etc. avoid such harmful effects in electronic =applications,. it is necessary .to have a means of identifying, tubes in an early stage of interface impedance growth to permit their removal from equipment well in advance of the time: at which they would cause failure.

It is, therefore, a primary object of the present invention to provide a means of testing and measuring. the cathode interface impedance in an electron tube.

It is a further object to provide a system for testing and measuring the resistance component of interface impedance which, is inherently simple, easy to operate and: which is readily applicable to rapid laboratory and industrial use.

In accordance with the present invention there is provided a method of determining the cathode interface impedance of an electron tube comprising the steps of applying a high frequency sign-a1 to the tube to determine the resistance of the tube at high frequency, applying a low frequency signal to the tube to determine the tube resistance at low frequency, the value of the difference between the two resistances being the value of the resistance component of the cathode interface imped'ance.

Also, in accordance with the present invention there is provided a circuit for, measuring cathode interface. impedance of 28. tube comprising high and low frequency generating means, switching means for applying signals from either of said generators to said tube and means for indicating the resistance of the tube at high or at low frequencies.

For a better understanding, of the present invention together with other (and further objects thereof, reference is had to the following description taken in connection with the accompanyingdrawings and its scope will be. pointed out in the appended claims.

-In .the drawings, Figs. 1-5 are schematic representation of circuits illustrating embodiments of the present invention.

In Figs. 1 and 2, like elements are designated by the same reference numbers. Similarly, in Figs. 3-5, like elements are designated by the same reference numbers.

Referring more particularly now to Fig. 1, there is shown a circuit which illustrates the basic principle of the invention and provides means for observing and measuring the resistance component .of cathode interface impedance present in an electron tube. The output of a pulse generator 2 which provides a rectangular pulse is connected to the plate 4-of an electron tube 6 through a variable resistor 8. Tube 6 may be of any type and in the present embodiment is a triode which is diode connected, the grid 10 being connected to plate 4 as shown. It is to :be understood of course that. any electron tube may be, tested by this method and that the tube need not be tested in the circuit as a diode but characteristic electrode potentials may be used. A suitable switch 14 is connected across resistor 8 for shorting out resistance 8 in the circuit when desired. Cathode 16 of tube 6 is connected to ground through loadresisto'r 18; schematically, the interface impedance to be deter mined is represented in dotted lines by the parallel combination' 20 of a resistance component 22 shunted by a capacitance component 24 connected between cathode 16 and resistor 18. The output of tube 6 developed across resistor 18 is applied to the vertical plates of an oscilloscope 26, suitable for displaying pulse wave forms.

In operation, a rectangular pulse from pulse generator 2 is applied to. plate 4 with variable resistor-8 set. at zero ohms (shorted out) and the oscilloscope gain is adjusted Patented May 21, 1957 Tof to provide a convenient amplitude for viewing. The resulting trace on the cathode ray tube of oscilloscope 26 is indicated by the upper trace A, which shows an instantaneous peak current amplitude followed by an exponential decay to .a plateau or steady state amplitude indicated :by the dotted line C. The resistance in the circuit is then increased until the peak amplitude of a second trace B equals the plateau or steady state current amplitude C of trace A. The value of resistance 8 is then equal to the resistance component of the inter face impedance.

The following is a theoretical explanation of the above described operation. With variable resistor 8 at zero, the steady state current of upper trace A, is equal to the pulse voltage output from generator 2 divided by the sum of value of the tube static resistance, the resistance value of load resistor 18 and interface resistance component 22. For the first instant of the pulse, there is no voltage drop across interface resistance component 22 since it is bypassed by capacitance component 24,

and therefore the peak amplitude of trace A is the amplitude obtainable if there were substantially no interface resistance present in tube 6 whereas the steady state value of the amplitude of trace A, is the reduced amplitude caused by the presence of such resistance. Similarly, with a portion of resistor 8 in the circuit, the steady state amplitude of trace B is equal the pulse voltage output from generator 2 divided by the sum of the value of the tube static resistance, the amount of increase of resistance of variable resistor 8 from Zero, the value of fixed resistor 18 and interface resistance component 22. As described in connection with trace A, at the first instant of trace B there is no voltage drop across interface resistance component 22 since the current is bypassed by capacitance component 24. Since the peak amplitude of trace B is adjusted to the steady state amplitude of trace A, the resistance increase in variable resistor 8 from zero is equal to the interface resistance component. Theoretically, it is well known in the art that the leading edge of a rectangular pulse is made up of an infinite range of frequencies and is eifectively the equivalent of a high frequency signal.

As an example of a tube tested by this method, a sample of a tube type 25L6-GT gave values of interface resistance of 22 ohms at normal 25 volt heater rating and 260 ohms with 15 volts on the heater. The observed higher value of interface resistance measured at the lower cathode temperature is a known characteristic of interface impedance. Although the interface resistance is less at the higher cathode temperature, interface resistance grows more rapidly during operation at increased cathode temperature.

The value of interface capacitance component 24 can be calculated using a time constant t of the decay and the peak to plateau current ratio, as taken from the pulse waveform viewed on the oscilloscope when measuring the value of interface resistance. For convenience, capacitance 24 will be designated as Ci interface resistance 22 as R1, peak current of the trace as I-a and plateau current as Ib. A practical formula for the capacitance is therefore The peak to plateau current factor takes into account I At the first instant of the pulse, the current Ia flows through capacitance component 24 (none through resistance component 22) and the voltage across capacitance 24 builds up at a rate equal to its final voltage, divided by a time t, that would be required to attain this voltage at the initial charging rate. The final voltage is IbRi and the rate of change in voltage is R; t The latter divided into Ia results in the expression for Ci hereinabove set forth.

The time, t, is readily evaluated for an exponential shape of. decay as would occur for constant circuit parameters. It is the interval required for the decay to undergo 63.2% of its ultimate amplitude variation (amplitude method) or the time from the point of a tangency of a line tangent to the initial portion of the pulse waveform to the time where this line intersects a horizontal axis at plateau height (slope method).

In Fig. 2, there is shown a modification of the circuit of Fig. 1 wherein limitations on measurement sensitivity that are introduced because of inadequacies of the oscilloscope response and the pulse generator waveform are minimized, and where low values of interface resistance are measurable. The circuit inconporates a conventional difference amplifier which produces an output signal that is the difference between the voltage across the load of the tube under test and a predetermined fraction of the input pulse. By this means it is possible to cancel substantially all but the exponential type of decay produced by the interface impedance of the tube under test and any slight irregularities present in the pulse generator Waveform thus tend to .be eliminated because substantially the same irregularities are applied to the input terminals of the difference amplifier and produce a constant difference. The decay pulse can be amplified to the desired size for convenience in viewing on the oscilloscope Referring now to Fig. 2, the output of generator 2 which provides a rectangular pulse is applied to attenuator 30 whose output in turn is simultaneously applied to plate 4 of diode connected tube 6 through variable resistance 8 and to a diode connected tube 34 through the center arm of potentiometer 32. As in Fig. l, a suitable switch 14 is provided for shorting out resistance 8 when desired and the cathode 16 of tube 6 is connected to ground through load resistor 18. The cathode output of tube 6 is coupled to one input of a difference amplifier 36 and the output of tube 34 is coupled to the other input of-amplifier 36. By means of potentiometer 32 a variable fraction of the input pulse is obtained. Although a pure resistance may be used to couple the portion of the pulse from the potentiometer to the difference amplifier, diode connected tube 34 is preferably employed inwthe circuit instead of a pure resistance to provide a non-linear diode characteristic thereby simulating the characteristics of the tube under test and to com pensate for non-rectangularities in the pulse generator waveform. The output of diiference amplifier 36 is coupled to a cathode follower 38, the output of which is applied to oscilloscope 26. Since all of the block ele ments shown in'Fig. 2 are of construction and operation 3 well known in the art, it is believed that no specific circuitry is required.

The procedure for determining the value of the interface resistance is the same as that described hereinabove in connectionwith Fig. l, viz; obtaining a steady state amplitude for an upper trace A with resistance 8 shorted out. and then obtaining a lower trace B with a portion of resistance 8 in the circuit, trace B having a peak amplitude equal to the steady state amplitude of trace A. It is to be noted that the tubes used in the circuit other than the one being tested should be free of interface impedance. Also, since tube 34 operates at a low duty factor (practically at cut on), it is susceptible to rapid formation of interface impedance and therefore a tube type should be used which is designed to withstand cut off operation. The type 5963 tube-is one which can be used for this purpose.

Fig. 3 shows a circuit wherein the value of resistance component tube interface impedance of an electron tube may be determined by measuring the resistance of the tube when a high and a low frequency signal, respectively, is applied thereto, the difference between the two measured values being the value sought. The plate 40 of the tube 42 to be measured, which is diode connected, may be coupled through capacitors 48 and 50 to either a high frequency signal generator 52 or low frequency signal generator 54 respectively by means of suitable switches 56 and 64. The cathode 43 of tube 42 is connected to ground through a load resistor 58 and the output of tube 42 developed across resistor 58' is applied to a suitable A. C. meter 60 which may be scaled to read in ohms. A variable resistor 62 having one terminal 63 in circuit with cathode 43 is provided which may be inserted into the circuit in place of tube 42 by means of switch 64. Potential source 66 and resistance 68 is serially connected between the switches 56 and 64- and ground to properly bias plate 40in a positive direction. It is preferable to bias the plate at the operating point where the measurement of interface resistance is desired.

In considering the operation, a circuit is switched in by means of switches56 and 64 which includes high frequency signal generator 52, tube 42and load resistance 58 with potential source 66 and resistance 68 serially connected between switch 56 and ground to obtain a first reading on meter 60. Variable resistance 62 is now switched into the circuit in place of tube 40' by switch 64 and sufiicient resistance is inserted to obtain the same first meter reading. With said sufficient resistance 62 maintained in the circuit, low frequency signal generator 54 is switched into the circuit in place of high frequency signal generator 52 by means of switch 56 and the amplitude of the signal therefrom is adjusted to-provide the said first meter reading. Tube 42 is then switched into the circuit in place of resistance 62 and a second meter reading is thus obtained, the difference between. the second and. first meter reading being a function of the cathode interface resistance component.

An alternate method for utilizing the circuit of Fig. 3', is to commence with resistance 62 in the circuit set at a predetermined. value for example, 1000 ohms, with the amplitude of the signal for high frequency generator 52 adjusted to give a. reading of 1000 ohms on meter 60. The amplitude of. the signal from low frequency generator 54 is also adjustedto give the same 1000 ohms read ing on meter 60 when in. circuit with resistance 62. This completes the calibration of the meter and a tube may be inserted into the circuit. in place of resistance 62 by means of switch 64. The meter reading in ohms when the tube is in the circuit will give direct resistance readings in ohms for the tube at: highfrequency and low frequency respectively, the difference being equal to the interface resistance.

Fig. 4 is a circuit similar to that in Fig. 3, the difference being the presence of a parallel combination 70 comprising a variable resistor 72. and a high frequency choke coil, inductance 74 connected between resistance 68 and plate 40. Parallel combination 70 is provided for convenience since at low frequency, resistance 72 is effectively shorted out ofv the circuit and there is no need to insure that it. is set at zero. It is to be understood of course that a variable;- resistance may be used in place of parallel; combination. 70. In operation, the circuit first comprises, in series, low frequency generator 54, parallel combination 70, tube: 42 and resistance 58, with resistance 68 and potential source 66 being connected inseries between switch 56 and ground. The amplitude of the low frequency signal fromgenerator 54 is adjusted to provide a convenient meter reading preferably at the upper half of the scale. Variable resistance 62: is switched into the circuit in place of tube 42 by means of switch 64 andits resistance value is adjusted to give the same meter reading. High. frequency signal generator 52, is

then switched into the circuit in place of low frequency signal generator 54 and the amplitude of the signal means of switch 64, tube 42 is switched into the circuit in place of resistance 62 and resistor 72 is adjusted to give said same meter reading. The value of adjusted resistor 72 is equal to. the resistance component of the cathode interface impedance.

Fig. 5 is a circuit similar to those shown in Figs. 3 and 4 and is designed to compensate for tube capacitance in the final value derived for the interface resistance. Plate 40 of diode connected tube to be measured 42 may be connected through parallel combination 70 either to high frequency signal generator 52 or low frequency generator 54 through coupling capacitors 48 or 50, respectively, by means of switch 56. Cathode 43 is connected to. ground through load resistance 58 and the output developed across resistor 58 is applied to. meter 60. Variable resistance 62. may be switched into the circuit in parallel combination with tube 42 or removed from the circuit by means of switch 65. Connected to switch: 56 is a resistor 68'which may be connected to ground through a positive potential source 66 or negative potential source 67, by means of switch 69.

By means of switches 56 and 69, the circuit of Fig. 5 first comprises inseries low frequency signal generator 54, parallel'combination 70, diode connected tube 42, load resistor 58 and plate positive potential source 66 in series with resistor 68 which is connected to switch 56. The amplitude of the low frequency signal is adjusted and a suitable meter reading preferably at the upper end of the scale is then obtained. Then by means of switch 65, variable resistor 62 is switched into the circuit to form a parallel-combination with tube 42, and simultaneously, negative plate potential bias source 67 is switched into the circuit in place of potential source 66 whereby tube 42 is rendered non-conductive. Resistor 62 is then adjusted to give said same meter reading. High frequency signal generator 52' is then switched into the circuit in place of generator 54, resistance 72 is set at zero and the amplitude of the high frequency signal is adjusted to give said same meter reading. Then resistor 62 is switched out of the circuit, and simultaneously positive potential source 66 is switched into the circuit in place of potential source 67 and resistor 72 is adjusted. to. give said same meter reading. The value of adjusted resistor 72 is equal to resistance component of the cathode interface impedance.

To understand the operation of the circuits of Figs. 3, 4, and 5,. the equivalence of interface impedance and a capacitance-resistance circuit must be borne in mind. At. high frequencies, the effect of the interface impedance is to. act as a capacitance which readily permits the passage of an A. C. current whereas at low frequencies, capacitive reactance is high and there is a voltage drop across the resistance component of the internal impedance. Therefore effectively, at high frequencies, there is practically no interface impedance present in the tube whereas at low frequencies, there is. It follows, therefore, that at low frequencies, the resistance of a tube is equal to its internal resistance plus its cathode interface resistance and at high frequencies, it is equal to its internal resistance only the difference between the two being the cathode interface resistance. It is to be noted that in Figs. 4 and 5, parallel. combination 7i; has been utilized for convenience, as at low frequencies, resistance 72 is effectively shorted out and at high frequencies, inductance 74 prevents passage of the current therethrough.

Figs. 1 and 2 illustrate circuits applicable in testing tubes for cathode interface impedance when such tubes are adapted for use in class C operation whereas Figs. 3, 4 and 5 illustrate circuits for testing tubes adapted for class A operation. 7 It is to be understood that the values for interface impedance obtained by both methods need not be identical.

While there have been described what are, at present, considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made herein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A circuit for measuring the cathode interface impedance of an electron tube having an input and an output circuit wherein the presence of such impedance is indicated in said output circuit by the exponential decay from a peak to a steady state amplitude of a rectangular pulse applied to said input circuit, said impedance effectively comprising a parallel arrangement of a resistance and a capacitance in circuit with said cathode, comprising in combination a generator for applying rectangular pulses to said input circuit, a difference amplifier, means for applying a predetermined portion of said pulse to one input of said difference amplifier, said output circuit being coupled to the other input of said difference amplifier, means responsive to the output of said difference amplifier for indicating the waveform of a first output pulse therefrom and a variable resistance connected between the output of said generator and said input circuit for producing a second output pulse having a peak value equal to the steady state amplitude of said first output pulse, the value of the resistance being equal to the resistance component of said interface impedance.

2. An apparatus for determining the resistance of the cathode interface impedance of an electron tube having at least a cathode and an anode constituting input and output electrodes, said impedance effectively comprising a resistance shunted by a capacitance inserted in the cathode lead of said tube comprising means for applying a first voltage of a known given wave form between said anode and said cathode to produce a first current therethrough, the shape of the waveform of said voltage being such that for at least a given instant, the equivalent resistance of said cathode interface impedance is negligible relative to the reactance of the equivalent capacitance thereof, the resistance of said tube at said first given instant being equal to the magnitude of said first voltage at said first given instant divided by the first current through said tube at said first given instant, means for applying a second voltage of a known given waveform between the anode and cathode of said tube to produce a second current therethrough, the shape of the waveform of said second voltage being such that for at least a second given instant, the reactance of said equivalent capacitance is negligible relative to the equivalent resistance thereof, the resistance of said tube at said second given instant being equal to the magnitude of said second voltage at said second given instant divided by the second current through said tube at said second given in stant, means connected to an output electrode of said tube for indicating said first and second currents through said tube at said first and second given instants respectively whereby the equivalent resistance of the cathode interface impedance is equal to the difference between the resistance of the tube at said first and second given instants.

3. An apparatus as defined in claim 2, wherein said first voltage comprises a potential having a given low frequency component and said second voltage comprises a potential having a given high frequency component.

4. An apparatus as defined in claim 3, further comp-rising means for selectively applying voltages between said anode and said cathode.

5. An apparatus as defined in claim 4, further comprising means for replacing said tube in said apparatus with an adjustable resistance.

6. An apparatus as defined in claim 5, further comprising an adjustable resistance between said means for applying said voltages and said tube.

7. A11 apparatus as defined in claim 2 wherein said first voltage comprises a potential having a given low frequency component and said second voltage comprises a potential having a given high frequency component and said apparatus further comprises means for inserting an adjustable resistance in parallel arrangement with said tube.

8. Means for measuring the cathode interface impedance of an electron tube having at least a cathode and an anode, wherein the presence of said impedance is indicated by the exponential decay from a peak to a steady state amplitude of a square wave pulse applied thereto, said impedance effectively comprising a parallel arrangement of a resistance and capacitance in circuit with said cathode, comprising a square wave pulse source, a load circuit connected thereto, said load circuit comprising a series arrangement of an adjustable resistance which can be varied between a resistance value of zero and a value greater than the resistance component of said interface impedance, the anode cathode path of said tube, and a fixed resistance, and means for indicating the shape of a voltage developed across said fixed resistance whereby when the value of said adjustable resistance is such that said developed voltage has an initial peak amplitude which is equal to the steady state amplitude of a developed voltage when said adjustable resistance is zero, said adjustable resistance value is equal to the value of the resistance component of said interface impedance.

9. A method of determining the cathode interface impedance of an electron tube having at least an anode and a cathode, said impedance effectively comprising a parallel arrangement of a resistance and a capacitance in circuit with said cathode comprising the steps of applying a first rectangular pulse directly as an input to the anode of said tube to produce a first current pulse through said tube having a first initial peak amplitude whichdecays exponentially to a first steady state amplitude, applying the resulting output to an indicating device to obtain an indication of said first steady state amplitude, applying a second rectangular pulse which is identical to said first applied pulse through a variable resistance as an input to said tube to produce a second current pulse through said tube having a second initial peak amplitude which decays exponentially to a second steady state amplitude, applying the resulting output to an indicating device to obtain an indication of said second initial peak amplitude, adjusting said resistance so that said second peak amplitude is equal to said first steady state amplitude whereby the value of said adjusted resistance is equal to the value of the resistance component of said interface impedance.

10. A method of determining the cathode interface impedance of an electron tube having at least an anode and a cathode, said impedance efiectively comprising a parallel arrangement of a resistance and a capacitance in circuit with said cathode comprising the steps of applying a square wave signal directly as an input to the anode of said tube to produce a first current pulse through said tube having a first initial peak amplitude which decays exponentially to a first steady state amplitude, developing a first potential proportional to said first current pulse, deriving a first voltage pulse whose amplitude is the difierence between the amplitude of said first potential and a given portion of said square wave signal, applying said first voltage pulse to an indicator to indicate the steady state amplitude of said first voltage pulse, applying said square wave signal through a variable resistance as an input to said tube to produce a second current pulse through said tube having a second initial peak amplitude which decays exponentially to a second steady state amplitude, developing a second potential proportional to said second current pulse, deriving a second voltage pulse whose amplitude is the difference between 9 10 said second potential and said given portion of said sig- References Cited in the file of this patent nal, applying said second voltage pulse to an indicator to indicate the initial peak amplitude of said second voltage UNITED STATES PATENTS pulse, adjusting said variable resistance so that said sec- 2,412,231 1946 ond voltage pulse has an initial peak amplitude equal 5 2,4437% MacNlchol June 22, 1943 to the steady state amplitude of said first voltage pulse 2,616,058 Wagner 1952 whereby the value of said adjusted resistance is equal to the value of the resistance component of said interface impedance. 

